The long term goal of this study is to define the structural and compositional factors relating to the caries susceptibility and resistance of the tooth mineral. (The mineral phase of tooth enamel and dentine is a non-stoichiometric calcium hydroxyapatite, modified by the presence of minor constituents such as CO3=, Mg2 ion, HOP4=, Na ion). Studies on strucutural factors include: crystallite size and morphology, lattice parameters, internal strain; thermal stability. Studies on compositional factors will focus on the effects of some minor constituents and of some 'trace elements' (suspected of being either cariogenic or cariostatic) on some properties of biological and synthetic apatites. Biological materials will include dental enamel from teeth of different mammalian species; from human teeth of known ages; from human teeth of known 'trace element' contents obtained from regions of high and low caries incidence; from fluoridated (shark's teeth) and non-fluoridated fish enameloids. Synthetic apatites will be prepared from aqueous systems in the presence of various types and amounts of 'trace elements' or minor constituents. X-ray diffracion, infrared absorption spectroscopy, atomic absorption spectrophotometry, colorimetry, thermo-gravimetry (TGA), and in some cases, transmission (TEM) and scanning (SEM) electron microscopy, will be employed for characterization of the biological and synthetic materials.